-LRB- CNN -RRB- -- Amid a high alert over an alleged al Qaeda plot to attack cities in Europe , France has warned its citizens about travel to the United Kingdom , saying British authorities believe a terror attack is `` highly likely . ''

The move follows a United States warning that Americans be careful about traveling in Europe because of the risk of terror attacks .

Neither the UK nor France raised threat levels in response to the American warning , but each has now warned its citizens about travel elsewhere on the continent .

Europe remains on edge after the warnings , based at least partly on intelligence about a plot obtained from a German-Afghan in U.S. custody in Afghanistan .

Police in France seized 12 people for suspected terrorist ties Tuesday , but it is not clear whether they are connected to the plot that prompted the warning .

Those arrests came a day after a suspected drone strike in Pakistan left five German nationals dead , adding to the unease .

French national police said the 12 suspects were seized in the south of France .

Three have links to a French man -- identified as Riahd Hennouni -- arrested in Italy last month , authorities said . Two of them were arrested in Marseilles and the third was arrested in Bordeaux .

The nine other arrests happened throughout southern France , including some in Marseilles and at least one in Avignon , police said . Police in Marseille said the nine have links to an Islamist movement and are suspected of trying to obtain arms and explosives .

It is unclear whether the 12 are French citizens . None of them has been charged . Under French law , police can detain and question suspects for up to six days without filing charges .

Meanwhile , Italian police said Sunday they detained a French citizen of Algerian origin who is suspected of being a member of al Qaeda . The man was arrested last month by Italian police in Naples , Italy , on a European arrest warrant requested by France , the chief prosecutor in Naples told CNN Monday . The arrest was not announced at the time .

Didier Allard of France 's Palais de Justice appeals court identified the man as Hennouni and said an anti-terror judge in Paris had requested his arrest and asked that he be extradited from Italy .

Elsewhere , Pakistani intelligence confirmed that five German nationals were killed in a suspected U.S. drone strike Monday in the northwestern part of the country .

The Germans were among 11 suspected militants killed Monday in the Mir Ali area of North Waziristan , one of the seven districts of Pakistan 's tribal region -- a remote and rugged area where militants hold sway .

Three others were foreigners whose nationalities were not disclosed , said the officials , who did not want to be named . The other three were Pakistanis .

The bodies were removed by the Taliban , one of the officials said .

The militants killed are believed to have been members of the group Jihad al Islami , the officials said .

The news of the arrests in France and the Germans ' deaths in Pakistan come amid warnings from the United States , Britain , Sweden and Japan advising their citizens traveling abroad about potential terrorist attacks in Europe .

It is not known if Tuesday 's arrests and Monday 's drone strike , however , are related to the latest alerts .

The U.S. Department of Homeland Security and the FBI issued a joint bulletin Sunday warning of the potential attacks .

The reported plot prompted the U.S. State Department to issue a Europe-wide security advisory for Americans traveling abroad .

The alert did not cite specific countries because the information about the threat was not specific enough to do so , a State Department spokesman said Monday .

Jihadists from the German city of Hamburg are alleged to be at the heart of the suspected plots , European intelligence officials said .

Western intelligence officials say they learned about the potential attacks after Ahmed Sidiqi , a German citizen of Afghan descent , was arrested in Afghanistan in July and taken to the U.S. air base at Bagram for questioning .

He has not been charged , and intelligence sources in Germany said he was cooperating with the investigation .

According to German intelligence officials , Sidiqi and 10 others left Hamburg in 2009 for the tribal areas of Pakistan -- where most of the group joined a jihadist group fighting U.S. and coalition forces across the border in Afghanistan .

Sidiqi told American interrogators that at least one member of his travel group was to be a `` foot soldier '' in the plot , with other members of the group helping to plan the attacks , a European counterterrorism official told CNN .

Sidiqi 's father , Mohammed Naim Sidiqi , said his son was on his way to the German embassy in the Afghan capital to get a new passport when he was detained .

Sidiqi 's father says he does not believe the allegations that his son is at the heart of an al Qaeda conspiracy .

`` My son is not a killer , he 's just not capable of that , '' he told CNN , stressing that such acts are absolutely forbidden in Islam .

Sidiqi 's sister says she , too , is shocked by the reports .

According to Sidiqi 's sister , her brother left Hamburg last year to start a new life in Afghanistan with his wife , an Indonesian citizen .

She says that financial problems in Hamburg , including difficulty paying his rent , contributed to his decision to leave .

`` He had some debts and he was determined to earn money in Afghanistan to repay them , '' his father told CNN .

According to German intelligence agencies , Sidiqi had a job cleaning aircraft at Hamburg International Airport before leaving for Pakistan .

Sidiqi 's father said he also did some work in Germany related to computers .

Before setting off , Sidiqi told his family he was going to meet some friends in Pakistan and then go across from there to Afghanistan , according to his sister .

In later phone calls he would not reveal his location , according to his father .

Sidiqi 's sister confirmed he had attended the Taiba mosque in Hamburg , where several of the 9/11 conspirators met .

CNN 's Niki Cook , Winnie Andrews , Nic Robertson , Paul Cruickshank , Matiullah Mati and Frederik Pleitgen contributed to this report .

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French warning : British authorities believe an attack is likely

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U.S. , others have warned about travel in Europe because of terror threats

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U.S. warning based on intelligence from a German-Afghan in U.S. custody

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This week has seen arrests and suspected drone strikes against terror suspects